[Big Dave]

Finding a working 5.7 or 4.3 diesel will take some serious looking. They would bolt right up, though.

6.2s and 6.5s are readily available and are generally more reliable engines. Newer ones (still avaialble through Hummer dealerships) are quite a bit better than the old ones but at the price of increased weight. 1000 lb for new vs 700 lb for old. a well rebuilt 6.5 T would last forever in a Corvette. Again, these engines are bolt-ups. The Vette's T-56 has a 0.5:1 ratio top gear that can take advantage of the tremendous torque of the 6.5 or 6.2. 70 MPH @ 1200 RPM. GM diesel guys will tell you that the 6.2 - with its smaller pre-cups - gives better MPG but these engines have cooling issues.

Putting a 6.5 in a C5 Corvette would be a fairly easy engine swap project as such projects go. The gearing would be optimized from the git-go and a perusal of Bonneville 'Vettes would give good insight into aero improvements. No point re-inventing the wheel if somebody else has already done the work.